The introduction of Spartan® herbicide (Sulfentrazone) in the 1990s revolutionized sunflower production in the U.S. Spartan gave sunflower producers the needed tool to grow this crop in a no-till system successfully for the first time. A few years later Dual Magnum® (S-metolachlor) was labeled on sunflower as well, but without a lot of fanfare. That was largely due to the product being weak on kochia, the “Darth Vader” of troublesome sunflower weeds.

For the 2012 season, a new herbicide will be available for sunflower producers that merges these two active ingredients into one product. The product, called Broadaxe®, is expected to be labeled in late 2011 or early 2012. The product owner, FMC, is planning a full launch for the 2012 season.

FMC has been looking at this combination for the last five years in all the sunflower production states. The company noticed that Broadaxe really stuck out as demonstrating superior weed control, according to Sam Lockhart, technical support specialist for FMC. The idea is not entirely new. In the October/November 2010 issue of The Sunflower, a producer near Dodge City, Kan., reported his use of a mixture of the two labeled herbicides to control palmer amaranth.

The advantage of Broadaxe over conventional Spartan formulations is longer residual, better control of early season grasses such as green, yellow and giant foxtail and barnyardgrass. It also provides a broader control spectrum of several key broadleaf weeds, including pigweed species (e.g., palmer amaranth), lambsquarter species and Russian thistle.

University researchers like Drs. Brian Jenks (NDSU), Richard Zollinger (NDSU) and Phil Stahlman (KSU) like what they have been seeing in their Broadaxe research plots.

For Jenks, who is located at the NDSU North Central Research Extension Center at Minot, N.D., the product has done a good job of controlling foxtail that may be resistant to Group 1 and Group 3 herbicides. In addition, he has observed good control of lambsquarters, pigweed and wild buckwheat when applied preplant or pre-emergence.

Stahlman, located at the KSU-Hays Research and Extension Center, has seen good control of foxtail as well as stinkgrass, fall panicum and witchgrass, along with partial control of longspine sandbur, wooly and prairie cupgrass, shattercane and Johnsongrass seedlings (but not rhizome Johnsongrass).

Another advantage that Stahlman has observed with Broadaxe is improved control of kochia. He notes that glyphosate-resistant kochia is well documented in Kansas and other regions. Kochia resistance to ALS and triazine herbicides has been around for some time. The combination of Broadaxe with two active ingredients is an important tool in resistance management.

All three scientists note enhanced synergy with the two ingredients combined. NDSU’s Zollinger worked with the product for the first time in the 2011 season and saw nearly complete control of foxtail, barnyardgrass, wild mustard, redroot pigweed, lambsquarters, eastern black nightshade, biennial wormwood and marshelder as well as partial control of dandelion. Some of the weed control was better with the mixture compared to Spartan or Dual alone. “There must be some synergy with the mixture,” Zollinger states.

Rates, Incorporation & Soil Types

All of the university researchers found that the higher rates did the best job. Brian Jenks says “weed control was typically better with the 35-oz rate compared to the 25-oz rate.” Rich Zollinger agrees with the higher rate recommendation. Phil Stahlman likes the higher rates to get the longest season control of difficult weeds like palmer amaranth, which can germinate throughout the growing season.

Broadaxe can be applied 14 days prior to planting and up to three days after planting. Like Spartan and Dual, Broadaxe needs rainfall for activation. Generally, a half inch is needed for good activation in no-till or conventional till. Both Zollinger and Jenks like a pre-emergence application to get the longest residual. Planting time rainfall has not been a problem in the last several years in the Dakotas/Minnesota.

Zollinger thinks abundant moisture is the key reason why the results were so good in his 2011 trials. He is interested in seeing the level of weed control in a dry spring.

Stahlman suggests a seven- to 10-day preplant application in Kansas dryland production with a mixture of glyphosate. “Our spring rainfall is so unpredictable. It is important to have the product on the ground for a longer period of time to ensure a rainfall event,” he says

FMC’s Sam Lockhart cautions growers to not cover Broadaxe with much soil when planting. “Broadaxe will have trouble controlling weeds if it is buried under a lot of soil,” he says. “It is this top layer where the small-seeded weed species will be located when they germinate.” KSU’s Stahlman also warns about displacing treated soil within the row in a preplant situation when no moisture has been received.

The labeled rates will range from 17 to 38 oz/acre. Lockhart says that lower rates will be necessary on very light soils with low organic matter and high pH. “It will be important for growers to consider soil sampling or grid sampling their ground to dial in a proper rate based on the rate chart on the label,” he notes. “If a grower knows his soil type, soil organic matter and soil pH, it will be easier to pick a rate to use.”

Lockhart and the university researchers all agree that the product has great potential for added weed control in sunflower. There are weaknesses in volunteer grains, wild oat and some large-seeded broadleaf weeds. But up to this point, researchers have been impressed with the results.